We will extend our studies on DNA-protein interactions between the bacteriophase lambda repressor and cro proteins and operator DNA. The X-ray crystal structures of the amino-terminal domain of repressor (chich binds specifically to the lambda operators) and of cro suggesst detailed molecular models of the binding of these proteins to DNA. We will use synthetic oligodeoxyribonucleotides as mismatch-mutagens to creat specific mutations in the genes encoding repressor and cro. Mutant proteins will be characterized in vitro to determine their binding affinities for both wildtype and chemically or mutationally altered operators. These studies will clarify the role of various amino acid residues in making specific protein-DNA contacts. The same experimental approach will be used to study the amino acids apparently involved in the interaction of repressor with TNA polymerase which is required for transcriptional activation of PRM. We will also begin crystallographi and mutational studies of the bacteriophage 434 repressor and cro proteins which are functionally and structurally homologous to the lambda proteins.